Interstrand crosslinks are particularly dangerous DNA lesions as they are absolute blocks to replication and transcription. They are believed to occur as a product of oxidative metabolism, and are also a consequence of treatment with some chemotherapy drugs. Psoralens are photoactive DNA interstrand crosslinkers that have been used clinically for many years. We synthesized, and demonstrated the activity of, antigen linked psoralens. Photoactivation of defined subnuclear regions in cells incubated with the compounds resulted in localized crosslinks. Repair of these adducts was followed in repair proficient and deficient cells. We are using this approach to follow the recruitment of proteins into sites of crosslink repair.